During a contraction the circumference decreases in length more than the distance between the apex to base and the heart changes shape to one with a greater surface-to-volume ratio. Since this implies a difference in tension and velocity of shortening in different parts of the heart all cardiac cells cannot function in exactly the same way during a contraction. Preliminary evidence suggests that the cause of this difference lies outside the myocardial cell. This project is concerned with examining the contributions of the connective tissue and the intercellular junctions to the heterogeneity of mechanical performance of different cardiac cells. Strips from frog and rat hearts in which the fibers run parallel and in which they do not run parallel will be isolated and their striation patterns continuously monitored. Two types of physiological experiments will be performed. In the first progressively larger volumes of the tissue will be activated to determine the influence of active cells on the length of adjacent passive cells as an indication of the degree of mechanical coupling between cells. In the second the resting and active sarcomere length-tension curves will be determined before and after either treatment with collagenase to determine the influence of connective tissue or treatment with hypertonic solutions to determine the influence of intercellular junctions on the mechanical properties of groups of cardiac cells. The results of these functional studies will be examined in light of the amount and orientation of connective tissue and the intercellular junctions as determined by light and electron microscopy. These studies should indicate whether connective tissue and/or intercellular junctions are responsible for the special characteristics of the contraction of individual myocardial cells in the heart.